Motor protector circuit

Abstract

A circuit useful to protect motors and other dynamoelectric machines from overtemperature conditions caused by a fault condition such as locked rotor, overcurrent, or the like. The circuit comprises first and second tuned branch circuits coupled to the power lines of the motor or other machine along with an oscillator circuit and detector circuit and a switch to change the effective impedance of one of the tuned circuits. The tuned circuits each includes a series connected inductance and capacitance while the switch, which can be an electro-mechanical device such as a heat responsive thermostatic switch, or a solid state switch such as an NTC or PTC thermistor is connected around one of the components of one of the tuned circuits. The oscillator may be a conventional one adapted to provide a selected frequency, preferably high relative to that of the motor power supply. The detector, which also may be of a conventional type and the oscillator are both coupled to one of the power lines of the motor by respective toroid cores through which the power line passes intermediate the tuned circuits. When the switch is in one state the tuned circuits will resonate enabling the detector to detect the signal and to give an output thereby effecting indication of a fault condition or deenergization of the motor. When the switch is in its opposite state the tuned circuits will not resonate, and there will be no output signal given by the detector.

What is claimed: 
     
       1. A circuit for detecting overtemperature conditions in a load, comprising: first and second AC power supply lines for energizing a load,   tuned circuit means connected across the first and second power lines,   oscillator circuit means for impressing a signal having a selected frequency in said tuned circuit means,   detector circuit means for detecting said signal,   power supply means coupled to said oscillator circuit means and detector circuit means,   thermally responsive switch means having first and second states electrically coupled to the tuned circuit means so that in one state the impedance of the tuned circuit means is at a level precluding resonance therein while in the other state the impedance of the tuned circuit means is at a level causing resonance therein, the thermally responsive switch adapted to be thermally coupled to the load whereby overtemperature conditions in the load will cause the thermally responsive switch to change from one state to the other, and   output means coupled to said detector circuit means providing an output signal when the tuned circuit means is in resonance.   
     
     
       2. A circuit according to claim 1 in which the tuned circuit means includes first and second tuned circuits each including an inductance and a capacitance. 
     
     
       3. A circuit according to claim 1 in which the thermally responsive switch is a bimetal switch connected in parallel with one of the inductance and capacitance is one of the tuned circuits. 
     
     
       4. A circuit according to claim 1 in which the thermally responsive switch is a PTC thermistor connected in parallel with one of the inductance and capacitance in one of the tuned circuits. 
     
     
       5. A circuit according to claim 2 in which the thermally responsive switch is a bimetal switch connected in parallel with one of the inductance and capacitance in one of the tuned circuits. 
     
     
       6. A circuit according to claim 2 in which the thermally responsive switch is a PTC thermistor connected in parallel with one of the inductance and capacitance in one of the tuned circuits. 
     
     
       7. A circuit according to claim 2 in which the thermally responsive switch is an NTC thermistor connected in parallel with one of the inductance and capacitance in one of the tuned circuits. 
     
     
       8. A circuit according to claim 2 further including a second tuned circuit means connected across the first and second power lines, a second oscillator circuit means including third and forth tuned circuits for imposing a second signal having a selected frequency different from the signal provided by the first oscillator means, a second detector circuit means for detecting said second signal,   power supply means coupled to said second oscillator circuit means and second detector circuit means,   and a second thermally responsive means having first and second states electrically coupled to the second tuned circuit means so that in one state the impedance of the tuned circuit means is at a level precluding resonance therein while in the other state the impedance of the second tuned circuit means is at a level causing resonance therein, the second thermally responsive switch adapted to be thermally coupled to the load whereby overtemperature conditions in the load will cause the second thermally responsive switch to change from one state to the other, and   second output means coupled to said second detector circuit means providing an output signal when the second tuned circuit means is in resonance.   
     
     
       9. A circuit according to claim 8 in which the said thermally responsive switches change from one state to the other upon reaching approximately the same temperatures. 
     
     
       10. A circuit according to claim 8 in which the said second thermally responsive switch changes from one state to the other upon reaching a temperature substantially different than the temperature at which the other thermally responsive switch changes from one state to the other. 
     
     
       11. A circuit according to claim 8 in which the thermally responsive switches are bimetal switches. 
     
     
       12. A circuit according to claim 8 in which the thermally responsive switches are PTC thermistors. 
     
     
       13. A circuit for detecting overtemperature conditions in a motor comprising: first and second AC power supply lines for energizing a motor,   a tuned circuit comprising a series connected inductance and capacitance connected across the first and second power supply lines,   a tuned sensing circuit comprising a series connected inductance and capacitance connected across the first and second power supply lines intermediate the motor and the tuned circuit,   oscillator circuit means coupled to one of the power lines for impressing a signal at a selected frequency in said one line,   detector circuit means coupled to one of the power supply lines for detecting said signal, the oscillator circuit means and the detector circuit means disposed intermediate the tuned circuit and the tuned sensing circuit,   power supply means coupled to the oscillator circuit means and the detector circuit means,   a thermally responsive switch coupled to the tuned sensing circuit to change the effective impedance of the tuned sensing circuit from a level precluding resonance in the tuned circuits at temperatures below a preselected level to a level causing resonance in the tuned circuits at temperatures above the preselected level, and   output means coupled to said detector circuit means providing an output signal when the tuned circuits are in resonance.